Please refer to FIG. 1(a), which is a top view of the micro actuated blazed grating according to the prior art. As shown in FIG. 1(a), in order to own the following properties, such as being performed as an optical switch, having a blazed grating shape, and maintaining the grating shape after being actuated, a conventional micro actuated blazed grating mainly includes the silicon substrate 1, the structural pillar 2, the torsion bar 3, and the suspended grating mirror 4.
Please refer to FIGS. 1(b)–(c), which are the schematic diagrams showing the employments of the micro actuated blazed gratings in FIG. 1(a). As shown in FIG. 1(b), the reflective light 6 is generated by reflecting the incident light 5 with the grating mirror 4 when no voltage is applied to the micro actuated blazed grating. As shown in FIG. 1(c), when a voltage is applied to the micro actuated blazed grating, the grating mirror 4 is twisted at a rotative angle via the torsion bar 3 and the structural pillar 2. In which, the structural pillar 2 is used as the fulcrum of the rotation, and the rotative angle is called a blazed angle of the micro actuated blazed grating. Also, when the grating mirror 4 is twisted, the incident light 5 would be diffracted into the diffractive lights 7. According to the aforesaid design, the micro actuated blazed grating is able to be employed as an optical switch and perform the inherently physical properties as that of a common blazed grating.
The conventional manufacturing method of the micro actuated blazed grating is performed by three masks and the surface micromachining technology. Further, the manufacturing method can be divided into four parts: a) the manufacture of the lower electrodes, b) the manufacture of the structural pillars, c) the manufacture of the main body of the micro actuated blazed grating, and d) the release of the structures.
a) The manufacture of the lower electrode: A lower electrode is made from a silicon substrate and a silicon nitride. Preferably, the silicon substrate is a silicon wafer with a low resistance (<1 Ω-cm) so as to raise the conductivity of the lower electrode.
b) The manufacture of the structural pillar: The size of the structural pillar is determined by the size of the chosen mask, and the height thereof is determined by the thickness of the deposited thin film. In which, the height of the structural pillar equals to the thickness of the sacrificial layer. Further, the structural pillar is what is used for connecting the main body to the silicon substrate. The driving voltage of the micro actuator and the rotative angle of the grating mirror are determined by the height of the structural pillar (the thickness of the sacrificial layer). Further, the height of the structural pillar also plays a determinant role in the release process of the structures, since the structures would hardly be released when the thickness of the sacrificial layer (the height of the structural pillar) is not enough.
The relevant manufacturing methods of the structural pillar are schematically described as follows. After the lower electrode is manufactured, a sacrifice layer of aluminum or copper is formed on the silicon substrate by the plasma enhanced chemical vapor deposition (PECVD). Then, a photoresist is coated on the sacrifice layer and the positions of the structural pillars are defined by the mask. After developed, the phoresist having the defined positions of the structural pillars is processed under short-time hard bake with a high-temperature. After the hard bake, the phoresist is employed as the etch resist. Then, the sacrifice layer is etched by the reactive ion etch (RIE) method for forming the shapes of the structural pillars, and the photoresist is removed. After that, the low-stress silicon nitride is filled into the shapes of the structural pillars by the low-pressure chemical vapor deposition (CVD), and the structural pillars are accomplished accordingly.
c) The manufacture of the main body of the micro actuated blazed grating: The shape and the area of the main body are defined on the manufactured structural pillars by the third mask. After developed, the remained photoresist are removed. Then, a chromium layer regarded as an adhesive layer, and an aurum layer regarded as a conductive layer and an exposed layer are respectively coated thereon. After that, the structural shape of the main body and the area of the upper electrode are defined by the lift-off process. Afterwards, the main body of the micro actuated blazed grating is formed by etching with the RIE, in which the aurum layer is regarded as a cover layer.
d) The release of the structures: The last step for the elements manufactured by the surface micromachining technology is to release the structures and makes as them suspended. In which, the sacrifice layer is removed by the hydrofluoric acid solution. The relevant processing time is determined by the thickness of the sacrifice layer and the size of the main body of the micro actuated blazed grating.
However, the conventional manufacturing method of the micro actuated blazed grating has three defects as follows.
1) The performance of the sacrifice layer is unideal. The aluminum is always employed as the sacrifice layer in MEMS manufacture, since it is cheap, easy-obtained, and can be etched by acids and bases. On the other hand, since it is etched by acids and bases, it might be slightly etched by the developing agent as well during developed, which seriously affets the accuracy-needed MEMS structure. However, since the formula of the present developing agent has been changed and would not damage the aluminum, the above problem has been overcome. But another problem about the hydrogen formed during aluminum etched is still unsolved. Since the formed hydrogen has separated the aluminum from the etching agent, the inner of the aluminum layer might be etched incompletely. In addition, since the formation of the hydrogen will cause a force pressing on the inner structure of the grating, the structure of the grating might be damaged accordingly. As above, an aluminum sacrifice layer is unideal.
For solving the foresaid problems about the formation of the hydrogen, a copper is used to manufacture the sacrifice layer. At present, the etching solution for a copper sacrifice layer contains 8% (w/w) copper chloride and 8% (w/w) ammonium chloride. The advantage of the copper sacrifice layer is no hydrogen formation, but the disadvantage is that the etching rate is hardly under control. When treating a 2 μm×2 μm×2 μm area, the etching time for an aluminum sacrifice layer is 15 min, but the etching time for a copper sacrifice layer is 15–20 min. However, the etching time for completely removing the copper sacrifice layer is longer than that for forming the concavities of the structural pillars still. That is to say once the copper sacrifice layer is completely removed by etching, the sizes of the concavities of structural pillars will be too large. Nevertheless, the main structure of the grating and the torsion elements will be affected by the oversized cavities of the structural pillars. Therefore, a copper sacrifice layer is unideal, either.
2) The shape of the photoresit is changed during the period of short-time hard bake with high-temperature. The structural pillars are manufactured by a processing of defining it with a second mask, and the effects of the processing are the determinants to the appearances of the manufactured structural pillars and straightness of the torsion elements. In the manufacturing method of the structural pillars, the silicon oxide layer is etched by a reactive ion etching and the mask is used as a cover, and the mask should be treated under short-time hard bake with high-temperature first for hardening. However, since the shape of the photoresist is changed, the inner solvent of the photoresist would be volatilized quickly when being heated. Further, since the shape of the structural pillars is determined by the photoresis and the shape of the photoresist is changed in the baking process, the shape of the structural pillars is changed accordingly. For example, once the shapes of the edges and corners of the photoresist become very smooth, the appearance of the manufactured structural pillars would be changed accordingly. In other words, an undesired shape of the structural pillar might be formed due to the fact that the shape of the applied photoresist is changed.
3) The coatings of the chromium layer and the aurum layer are bad, so that the aurum layer might be lifted and the chromium layer might be etched by the hydrofluoric acid. It's known in Multi-User MEMS Processes (MUMPs) that the etching time for a silicon wafer in the hydrofluoric acid is 1.5–2.0 min. Generally, since the main body of the micro actuated blazed grating is a wide and long structure, the etching time for the main body of the micro actuated blazed grating is at least 8.5 min. However, during the etching of the main body of the micro actuated blazed grating, parts of the aurum layer would be lifted in 3–5 minutes, and therefore some parts of the chromium layer are etched by the hydrofluoric acid since then.
According to the above, the new manufacture method of micro actuated blazed grating and the structure thereof are the current subjects in the industry.